In most cases in which a fluid is being heated or cooled it is kept physically separate from the medium that is supplying heat to it or receiving heat from it by confinement within a vessel or conduit. Likewise, the medium supplying heat to or receiving heat from the fluid is also usually confined to the vessel or conduit. The vessels or conduits or both constitute a heat exchanger.
Most heat exchangers use a tube or a system of tubes as the barrier between the fluid being heated or cooled and the heating or cooling medium. Often it is necessary, in order to maximize the efficiency of the heat transfer, to provide a complicated system of baffles and tubes and to employ tubes that are constructed to enhance the rate of heat transfer, for example, by inclusion of ribs, fins, corrugations or the like. For durability fairly costly metals, such as copper, are often used. The complexity of the structure and the high cost of the materials make efficient, long-lasting heat exchangers very expensive.
Many devices that are in widespread use and employ heat exchangers have relatively low efficiencies. For example, residential furnaces and hot water heaters fueled by natural gas or oil have overall efficiencies of only about 50%. Natural gas and oil fired equipment could be made considerably more efficient using presently available technology in the design of heat exchangers, but only by considerably increasing the complexity and the size of the equipment and making it much more costly.